When you create/update records in a custom object the response you get back includes the status and the marketo GUID. You should be able to add additional fields like the dedupe field.
Forgive my ignorance of your use case, but is there any reason why you couldn't just run a follow-up object query based on the GUID returned to pull that? Moreover, when you do such a query it's not as though the dedupe field(s) are explicitly called out—you have to tell the query what field values you care about explicitly.
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